"Infrared Probes" consisting of an optically sensitive element that either projects into a process stream or into a beaker of fluid are well known Many of the probes now in use have an entry surface at the tip of the probe through which IR energy is directed at an angle that will give rise to internal reflection and an exit surface adjacent to the entry surface through which a portion of the energy that has been internally reflected down the rod and back exits to detectors. It is well known that because of the diverse paths that the various rays take, half of the energy exits from the exit face and half from the entrance face.
It is important in the operation of an infrared probe that the source is optically coupled to the entrance facet so that there is an efficient transfer of energy into the probe element and that the rays enter the facet at angles such that they will be internally reflected once within the probe element. A similar requirement for an efficient optical coupling exits to refocus emerging energy from the other facet onto the detector.
Means should be provided such that an infrared probe which projects into a beaker of fluid is always actively contacted by the same amount of fluid, otherwise absorption measurements will not be accurate It is highly desirable that such instruments not employ motors or other mechanical means, particularly if they are often to be carried from site to site.